WASHINGTON — The day after a deadly assault in Syria that bore many of the hallmarks of a chemical weapons attack, a sharply divided Obama administration on Thursday began weighing potential military responses to President Bashar al-Assad’s forces.

Senior officials from the Pentagon, the State Department and the intelligence agencies met for three and a half hours at the White House on Thursday to deliberate over options, which officials say could range from a cruise missile strike to a more sustained air campaign against Syria.

The meeting broke up without any decision, according to senior officials, amid signs of a deepening division between those who advocate sending Mr. Assad a harsh message and those who argue that military action now would be reckless and ill timed.

In an interview with CNN broadcast on Friday, Mr. Obama said the United States is “gathering information” about the chemical weapon reports, but he suggested that it is already clear that the incident will demand “America’s attention.”

“What we’ve seen indicates that this is clearly a big event of grave concern,” Mr. Obama said during an interview broadcast on CNN’s “New Day” program. “This is something that is going to require America’s attention.”

Mr. Obama said his administration does not expect cooperation from Syria’s regime to determine what happened in the attack. But he said that when chemical weapons are used, “that starts getting to some core national interests that the United States has.”

Debates about Syria have played out across the Atlantic. France backed the use of force to counter such an attack, and Turkey and Israel expressed outrage. But diplomats in several countries conceded there was no stomach among the Western allies, including the United States, for long-term involvement in a messy, sectarian civil war.

While the Obama administration said it would wait for the findings of a United Nations investigation of the attack, American officials spoke in strikingly tougher terms about what might happen if President Obama determined that chemical weapons were used.

“If these reports are true, it would be an outrageous and flagrant use of chemical weapons by the regime,” the State Department spokeswoman, Jen Psaki, said. “The president, of course, has a range of options that we’ve talked about before that he can certainly consider.”

The United States first confirmed in April that it believed the Syrian government had used chemical weapons, and Obama administration officials responded by signaling that they would supply the rebels with weapons. But to date, none have arrived, opposition officials said.

Among American officials, there was a growing belief that chemical weapons had been used in the latest attack, early Wednesday east of Damascus — potentially the worst of its kind since Saddam Hussein used chemical weapons against the Kurds in Iraq in the 1980s — and little doubt that anyone but Mr. Assad’s forces would have used them.

But given how difficult it was last time to prove the use of chemical weapons, administration officials offered no timetable for how long it might take this time, raising questions about how promptly the United States could act.

Israel said its intelligence strongly suggested a chemical weapons attack, while the Syrian opposition pointed to evidence, including the use of four rockets and the locations from which they were fired, that members of the opposition said proved the attack could have been carried out only by the government’s forces.

An opposition official described an assault that began shortly after 2 a.m., when the rockets, which they said were equipped with chemical weapons, were launched. Two were fired from a bridge on the highway from Damascus to Homs; the others were launched from a Sironex factory in the Qabun neighborhood of the Syrian capital. The Assad government has denied involvement, and Russian officials have accused the rebels of staging the attack.

The rebels said the government’s presumed goal was to weaken the opposition before a major conventional attack with tanks, armored personnel carriers and attack planes. On Thursday, fighting persisted in the area, raising doubts about the ability of the United Nations to send investigators to collect samples from the wounded and dead.

Among the options discussed at the White House, officials said, was a cruise missile strike, which would probably involve Tomahawks launched from a ship in the Mediterranean Sea, where the United States has two destroyers deployed.

The Pentagon also has combat aircraft — fighters and bombers — deployed in the Middle East and in Europe that could be used in an air campaign against Syria. The warplanes could be sent aloft with munitions to be launched from far outside Syrian territory, which is protected by a respectable air defense system.

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The targets could include missile or artillery batteries that launch chemical munitions or nerve gas, as well as communications and support facilities. Symbols of the Assad government’s power — headquarters and government offices — also could be among the proposed targets, officials said.

As leaders digested the harrowing images from Syria of victims gasping for breath or trembling, there was a flurry of phone calls among diplomats expressing horror at the calamitous situation in Syria and frustration at the lack of an obvious response.

“They are all bad choices,” said a European diplomat who asked not to be named because of the delicacy of the situation.

Secretary of State John Kerry spoke to Ahmad al-Jarba, the president of the Syrian opposition. Mr. Kerry expressed his condolences and the Obama administration’s “commitment to looking into what has happened on the ground,” Ms. Psaki said.

Mr. Kerry also spoke to Laurent Fabius, the foreign minister of France, who raised the prospect of military action. He called Turkey’s foreign minister, Ahmet Davutoglu; several Arab foreign ministers; the European Union’s senior foreign policy official, Catherine Ashton; and the secretary general of the United Nations, Ban Ki-moon.

At home, however, officials said the administration remained divided about how to proceed. “There’s a split between those who feel we need to act now and those who feel that now is a very bad time to act,” said one senior official, who spoke on the condition of anonymity to discuss the administration’s internal deliberations.

The official did not give details about who pushed for a hawkish response to the attack and who urged caution, but he said that some at the White House meeting raised worries that it would take time to build international support for a military response, and that any strikes against Mr. Assad’s government might worsen a refugee crisis that has already placed great strain on Syria’s neighbors, particularly Jordan.

Neither the United States nor European countries yet have a “smoking gun” proving that Mr. Assad’s troops used chemical weapons in the attack, the official said. But he said intelligence agencies had amassed circumstantial evidence that some kind of chemical had been used — not the least of which was the hundreds of casualties.

“The sheer number of bodies is one pretty good indicator,” he said.

If the United States decided to move ahead with a military response with the help of Western allies, it would almost certainly be one more in a lengthening series of conflicts where the United States has played a leading role that would lack the United Nations imprimatur, including in Kosovo, Afghanistan and Iraq.

Mohammad Salaheddine, a Syrian reporter for Al Aan Television, an Arab satellite television station, painted a bleak picture of the situation in East Ghouta, the area near Damascus that was the site of the reported chemical attack.

Mr. Salaheddine was reached by Skype on Thursday with the help of the Syrian Support Group, an American-based organization that backs the opposition.

He asserted that more than 1,500 people had been killed by the chemical attack and that many more had been wounded. The area, he said, was cut off by the fighting, making it hard for opposition members to smuggle hair, urine and blood samples out for analysis.

Adding to the opposition’s frustration, two of its officials said that none of the weapons American officials said would be provided by the C.I.A. had yet been delivered.

Senior United States military officials, in particular Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, have cited the risks and costs of a large-scale military intervention as has been urged by some members of Congress.

Yet the greater political risk now may be to Mr. Obama’s credibility, analysts said, given that he laid down a red line last summer to prevent Mr. Assad from using chemical weapons again.

“Assuming that there was a large-scale chemical attack, it indicates that the regime has not been deterred by the statements coming out of Washington,” said Jeffrey White, a former Middle East analyst with the Defense Intelligence Agency who is now a fellow at the Washington Institute for Near East Policy.

A version of this article appears in print on August 23, 2013, on Page A1 of the New York edition with the headline: Obama Officials Weigh Response to Syria Assault. Order Reprints|Today's Paper|Subscribe